CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		$ (8,587)	$ 53,932	$ 39,737
Cash flows from investing activities of continuing operations
Purchases of investments		(185,740)	(211,402)	(242,362)
Proceeds from sales of investments	[1]	107,368	132,183	141,470
Proceeds from maturities of investments		84,369	65,525	82,047
Change in deposits with banks		(19,290)	(25,311)	15,488
Change in loans		(58,062)	(39,761)	1,353
Proceeds from sales and securitizations of loans		8,365	18,140	9,610
Proceeds from significant disposals	[2]	3,411	265	5,932
Change in federal funds sold and resales		4,335	(17,138)	22,895
Payments due to transfers of net liabilities associated with significant disposals	[2],[3]	0	0	(18,929)
Changes in investments and advances—intercompany		0	0	0
Other investing activities		(2,984)	(2,089)	(2,621)
Net cash provided by (used in) investing activities of continuing operations		(58,228)	(79,588)	14,883
Cash flows from financing activities of continuing operations
Dividends paid		(3,797)	(2,287)	(1,253)
Issuance of preferred stock		0	2,498	6,227
Treasury stock acquired		(14,541)	(9,290)	(5,452)
Proceeds (repayments) from issuance of long-term debt, net		26,974	8,346	(8,224)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0	0
Change in deposits		30,416	24,394	8,555
Change in federal funds purchased and repos		14,456	(4,675)	(26,942)
Change in short-term borrowings		13,751	9,622	(37,256)
Net change in short-term borrowings and other advances—intercompany		0	0	0
Capital contributions from parent		0	0
Other financing activities		(405)	(316)	(428)
Net cash provided by (used in) financing activities of continuing operations		66,854	28,292	(64,773)
Effect of exchange rate changes on cash and cash equivalents		693	(493)	(1,055)
Change in cash and due from banks		732	2,143	(11,208)
Cash and due from banks at beginning of period		23,043	20,900	32,108
Cash and due from banks at end of period		23,775	23,043	20,900
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		2,083	4,359	4,978
Cash paid during the year for interest		15,675	12,067	12,031
Non-cash investing activities
Decrease in net loans associated with significant disposals reclassified to HFS		0	0	(9,063)
Decrease in investments associated with significant disposals reclassified to HFS		0	0	(1,402)
Decrease in goodwill and intangible assets associated with significant disposals reclassified to HFS		0	0	(223)
Decrease in deposits with banks with significant disposals reclassified to HFS				(404)
Transfers to loans HFS from loans		5,900	13,900	28,600
Transfers to OREO and other repossessed assets		113	165	276
Non-cash financing activities
Decrease in long-term debt associated with significant disposals reclassified to HFS		0	0	(4,673)
Consolidating adjustments
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		0	0	0
Cash flows from investing activities of continuing operations
Purchases of investments		0	0	0
Proceeds from sales of investments		0	0	0
Proceeds from maturities of investments		0	0	0
Change in deposits with banks		0	0	0
Change in loans		0	0	0
Proceeds from sales and securitizations of loans		0	0	0
Proceeds from significant disposals		0	0	0
Change in federal funds sold and resales		0	0	0
Payments due to transfers of net liabilities associated with significant disposals				0
Changes in investments and advances—intercompany		0	0	0
Other investing activities		0	0	0
Net cash provided by (used in) investing activities of continuing operations		0	0	0
Cash flows from financing activities of continuing operations
Dividends paid		0	0	0
Issuance of preferred stock			0	0
Treasury stock acquired		0	0	0
Proceeds (repayments) from issuance of long-term debt, net		0	0	0
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0	0
Change in deposits		0	0	0
Change in federal funds purchased and repos		0	0	0
Change in short-term borrowings		0	0	0
Net change in short-term borrowings and other advances—intercompany		0	0	0
Capital contributions from parent		0	0
Other financing activities		0	0	0
Net cash provided by (used in) financing activities of continuing operations		0	0	0
Effect of exchange rate changes on cash and cash equivalents		0	0	0
Change in cash and due from banks		0	0	0
Cash and due from banks at beginning of period		0	0	0
Cash and due from banks at end of period		0	0	0
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		0	0	0
Cash paid during the year for interest		0	0	0
Non-cash investing activities
Decrease in net loans associated with significant disposals reclassified to HFS				0
Decrease in investments associated with significant disposals reclassified to HFS				0
Decrease in goodwill and intangible assets associated with significant disposals reclassified to HFS				0
Decrease in deposits with banks with significant disposals reclassified to HFS				0
Transfers to loans HFS from loans		0	0	0
Transfers to OREO and other repossessed assets		0	0	0
Non-cash financing activities
Decrease in long-term debt associated with significant disposals reclassified to HFS				0
Citigroup parent company | Reportable legal entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		34,940	12,777	27,825
Cash flows from investing activities of continuing operations
Purchases of investments		0	0	0
Proceeds from sales of investments		132	3,024	0
Proceeds from maturities of investments		0	234	237
Change in deposits with banks		0	0	0
Change in loans		0	0	0
Proceeds from sales and securitizations of loans		0	0	0
Proceeds from significant disposals		0	0	0
Change in federal funds sold and resales		0	0	0
Payments due to transfers of net liabilities associated with significant disposals				0
Changes in investments and advances—intercompany		(899)	(18,083)	(35,548)
Other investing activities		0	0	3
Net cash provided by (used in) investing activities of continuing operations		(767)	(14,825)	(35,308)
Cash flows from financing activities of continuing operations
Dividends paid		(3,797)	(2,287)	(1,253)
Issuance of preferred stock			2,498	6,227
Treasury stock acquired		(14,541)	(9,290)	(5,452)
Proceeds (repayments) from issuance of long-term debt, net		6,544	7,005	127
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0	0
Change in deposits		0	0	0
Change in federal funds purchased and repos		0	0	0
Change in short-term borrowings		49	(164)	(845)
Net change in short-term borrowings and other advances—intercompany		(22,152)	4,620	9,106
Capital contributions from parent		0	0
Other financing activities		(405)	(316)	(428)
Net cash provided by (used in) financing activities of continuing operations		(34,302)	2,066	7,482
Effect of exchange rate changes on cash and cash equivalents		0	0	0
Change in cash and due from banks		(129)	18	(1)
Cash and due from banks at beginning of period		142	124	125
Cash and due from banks at end of period		13	142	124
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		(3,730)	351	111
Cash paid during the year for interest		4,151	4,397	4,916
Non-cash investing activities
Decrease in net loans associated with significant disposals reclassified to HFS				0
Decrease in investments associated with significant disposals reclassified to HFS				0
Decrease in goodwill and intangible assets associated with significant disposals reclassified to HFS				0
Decrease in deposits with banks with significant disposals reclassified to HFS				0
Transfers to loans HFS from loans		0	0	0
Transfers to OREO and other repossessed assets		0	0	0
Non-cash financing activities
Decrease in long-term debt associated with significant disposals reclassified to HFS				0
CGMHI | Reportable legal entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		(33,359)	20,662	12,336
Cash flows from investing activities of continuing operations
Purchases of investments		(1)	(4)	(4)
Proceeds from sales of investments		0	0	53
Proceeds from maturities of investments		0	0	0
Change in deposits with banks		11,861	(3,643)	(8,414)
Change in loans		0	0	0
Proceeds from sales and securitizations of loans		0	0	0
Proceeds from significant disposals		0	0	0
Change in federal funds sold and resales		9,730	(15,293)	8,037
Payments due to transfers of net liabilities associated with significant disposals				0
Changes in investments and advances—intercompany		(2,790)	(5,574)	1,044
Other investing activities		(24)	0	(101)
Net cash provided by (used in) investing activities of continuing operations		18,776	(24,514)	615
Cash flows from financing activities of continuing operations
Dividends paid		0	0	0
Issuance of preferred stock			0	0
Treasury stock acquired		0	0	0
Proceeds (repayments) from issuance of long-term debt, net		4,909	5,916	(139)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(2,031)	(9,453)	12,557
Change in deposits		0	0	0
Change in federal funds purchased and repos		5,748	3,236	(27,442)
Change in short-term borrowings		2,212	1,168	(1,737)
Net change in short-term borrowings and other advances—intercompany		3,931	680	4,054
Capital contributions from parent		(748)	5,000
Other financing activities		0	0	0
Net cash provided by (used in) financing activities of continuing operations		14,021	6,547	(12,707)
Effect of exchange rate changes on cash and cash equivalents		0	0	0
Change in cash and due from banks		(562)	2,695	244
Cash and due from banks at beginning of period		4,690	1,995	1,751
Cash and due from banks at end of period		4,128	4,690	1,995
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		678	92	175
Cash paid during the year for interest		4,513	3,115	2,346
Non-cash investing activities
Decrease in net loans associated with significant disposals reclassified to HFS				0
Decrease in investments associated with significant disposals reclassified to HFS				0
Decrease in goodwill and intangible assets associated with significant disposals reclassified to HFS				0
Decrease in deposits with banks with significant disposals reclassified to HFS				0
Transfers to loans HFS from loans		0	0	0
Transfers to OREO and other repossessed assets		0	0	0
Non-cash financing activities
Decrease in long-term debt associated with significant disposals reclassified to HFS				0
Other Citigroup subsidiaries and eliminations | Reportable legal entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		(10,168)	20,493	(424)
Cash flows from investing activities of continuing operations
Purchases of investments		(185,739)	(211,398)	(242,358)
Proceeds from sales of investments		107,236	129,159	141,417
Proceeds from maturities of investments		84,369	65,291	81,810
Change in deposits with banks		(31,151)	(21,668)	23,902
Change in loans		(58,062)	(39,761)	1,353
Proceeds from sales and securitizations of loans		8,365	18,140	9,610
Proceeds from significant disposals		3,411	265	5,932
Change in federal funds sold and resales		(5,395)	(1,845)	14,858
Payments due to transfers of net liabilities associated with significant disposals				(18,929)
Changes in investments and advances—intercompany		3,689	23,657	34,504
Other investing activities		(2,960)	(2,089)	(2,523)
Net cash provided by (used in) investing activities of continuing operations		(76,237)	(40,249)	49,576
Cash flows from financing activities of continuing operations
Dividends paid		0	0	0
Issuance of preferred stock			0	0
Treasury stock acquired		0	0	0
Proceeds (repayments) from issuance of long-term debt, net		15,521	(4,575)	(8,212)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		2,031	9,453	(12,557)
Change in deposits		30,416	24,394	8,555
Change in federal funds purchased and repos		8,708	(7,911)	500
Change in short-term borrowings		11,490	8,618	(34,674)
Net change in short-term borrowings and other advances—intercompany		18,221	(5,300)	(13,160)
Capital contributions from parent		748	(5,000)
Other financing activities		0	0	0
Net cash provided by (used in) financing activities of continuing operations		87,135	19,679	(59,548)
Effect of exchange rate changes on cash and cash equivalents		693	(493)	(1,055)
Change in cash and due from banks		1,423	(570)	(11,451)
Cash and due from banks at beginning of period		18,211	18,781	30,232
Cash and due from banks at end of period		19,634	18,211	18,781
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		5,135	3,916	4,692
Cash paid during the year for interest		7,011	4,555	4,769
Non-cash investing activities
Decrease in net loans associated with significant disposals reclassified to HFS				(9,063)
Decrease in investments associated with significant disposals reclassified to HFS				(1,402)
Decrease in goodwill and intangible assets associated with significant disposals reclassified to HFS				(223)
Decrease in deposits with banks with significant disposals reclassified to HFS				(404)
Transfers to loans HFS from loans		5,900	13,900	28,600
Transfers to OREO and other repossessed assets		$ 113	$ 165	276
Non-cash financing activities
Decrease in long-term debt associated with significant disposals reclassified to HFS				$ (4,673)

[1] Proceeds for 2016 include approximately $3.3 billion from the sale of Citi’s investment in China Guangfa Bank.

[2] See Note 2 to the Consolidated Financial Statements for further information on significant disposals.

[3] The payments associated with significant disposals result primarily from the sale of deposit liabilities.